252 OUTLINES OF BACTERIOLOGY. 



tank as an apparatus for the liquefaction of sewage solids has now been 

 universally acknowledged. 



The following classification of the various methods of dealing with 

 sewage biologically shows how different workers have grappled with 



Fig. 132. — Septic tank working under anaerobic conditions. (After Lafar.) 



the problem after obtaining a start by the invention of the cultivation- 

 and the septic-tanks. (Figs. 131, 132, 133.) 



1. Closed septic tank and contact beds. 



2. Open septic tank and contact beds. 



3. Chemical treatment, subsidence tanks and contact beds. 



4. Subsidence tanks and contact beds. 



5. Contact beds alone. 



6. Closed septic tank followed by continuous filtration. 



7. Open septic tank followed by continuous filtration. 



8. Chemical treatment, subsidence tanks and continuous filtration. 



9. Subsidence tanks followed by continuous filtration. 

 10. Continuous filtration alone. 



A contact bed (Fig. 133 b) is an artificial filter made up of such 

 materials as burnt clay, coke, cinders, gravel, etc. The bed is filled and 

 emptied alternately either with sewage after treatment in the septic 

 tanks (Nos. 1 and 2), or with sewage after chemical treatment (No. 3), 

 or without any preliminary treatment (Nos. 4 and 5). The object 

 of a contact bed is to hold the sewage in contact with the filtering 

 material as long as is necessary to effect oxidation, after which the 

 filter is emptied to allow a fresh supply of air to penetrate its inter- 

 stices. The action of such a filter is twofold. It separates mechanically 

 all the gross particles of suspended matter, and also effects the oxida- 

 tion of the organic matter, chiefly through the agency of the aerobic 

 bacteria. To secure the maximum efficiency the organisms must be 

 supplied with plenty of air, the filter must be supplied with a base such 

 as lime to neutralise the nitric acid that will be formed, and finally the 

 filtration must take place in the dark. The multiple contact beds 

 first used at Sutton is an example of the fifth method. At Sutton each 

 bed is 3J feet deep and the filtrant is burnt clay, which is occasionally 



